1) Field of the Invention
The present invention relates to medical devices and associated methods for treating various target sites within the body and, in particular, to medical devices and associated methods for fabricating and delivering medical devices that respectively include corrugated surfaces.
2) Description of Related Art
Vascular disease is common in the arterial system of humans. This disease often results in a build up of plaque or deposits on the vessel wall, which narrow the vessel carrying oxygenated blood and nutrients throughout the body. If narrowing should occur, for example, in an artery within the heart, blood flow may be restricted to the point of causing pain or ischemia upon body exertion due to the lack of oxygen delivery to the heart muscle. The flow disruption from a severe narrowing of the vessel or a plaque rupture may result in a blood clot formation and flow stoppage which, if occurring in the heart, would result in a heart attack.
Vascular disease may be anywhere in the body, and treating the disease is important to one's health. One method of treatment that is widely adopted is expanding the diseased narrowed sections of a vessel with an angioplasty balloon that is sized to the vessel's healthy diameter. The balloon is inflated to a high pressure to crack and expand the plaque outward, restoring the vessel diameter.
Another technique that may be used to treat the narrowing of a vessel is with a stent. A stent is a thin wall metal tubular member that can be expanded in diameter within the vessel to hold the ballooned segment open after the balloon is removed. Some stents (so-called “balloon-expandable” stents) are placed over a deflated angioplasty balloon and expanded by inflating the balloon, while other types of stents are self-expanding. Both types may be delivered to the treatment site by a catheter in a radially-collapsed configuration and then expanded within the diseased segment of the artery. Both types of stents may be fabricated by laser machining of thin wall metal tubes or may be fabricated from wires formed to a particular shape or by braiding wires into a tubular shape. Balloon-expandable stents are generally made from stainless steel or cobalt-containing alloys, where self-expanding stents tend to be made from highly elastic or pseudo-elastic metals, such as a shape memory nickel-titanium alloy commonly referred to as “Nitinol.”
Of particular interest in the design of stents is the amount of radial force that can be achieved for arterial support while minimizing the collapsed deliverable diameter. Stents must also be conformable, when expanded, to the curvature of the target artery segment, and should be flexible in bending in the collapsed deliverable diameter so that the stents can be passed through narrow tortuous arteries to the treatment site. In vessels that are close to the surface of the body, such as in carotid arteries, only self-expanding stents are considered suitable since the stent must spring back from an impact to the body and not close off the artery. Flexibility and good fatigue resistance are important properties for stents placed in arterial segments subject to flexure such as in joints.
Self-expanding tubular stents made of braided filaments of Nitinol wire are very useful due to their high flexibility and ability to be greatly reduced in diameter, by elongation of the braid, for delivery. The braided stents are even more flexible in their reduced diameter state. One limiting aspect of conventional braided Nitinol stents, however, is the ability to achieve high radial support compared to self-expanding stents cut from Nitinol tubing. To achieve greater radial support the braided tube may be fabricated from filaments having a greater diameter, but this increases the collapsed diameter profile and increases deliverable stiffness. An alternative to improve radial support is to heat set the braided stent at the desired expanded diameter with the helix angle of the filaments at a high angle relative to the longitudinal axis of the stent. This increases the length of the collapsed stent and increases the delivery force needed to push the stent through the delivery catheter since the filaments are under greater stress at a given collapsed diameter.
Another application of stents is in stent graft applications. One important application is the treatment of vascular aneurysms, a weakening and thinning of the vessel wall whereby the weakened area causes the vessel diameter to expand outward much like a balloon. The weakened wall is of greater risk of rupture due to pulsing blood pressure. Stent grafts are used to percutaneously reline the aneurysm, sealing against the proximal and distal healthy vessel wall and thus reducing risk of rupture by shielding the weakened wall from carrying the blood pressure. It is important that a seal be achieved on both ends of the graft against the arterial wall and that no leak occurs along the length of the graft. Such leaks would subject the weakened aneurysm wall to blood pressure. To achieve a seal, stent grafts have employed various design means to increase the pressure against the arterial wall at each end, such as incorporating end stents that have greater radial force, using thicker materials near the end, enlarging the expanded diameter of the stent graft at the ends, or adding sealing substances such as filler material. Most of these solutions increase the collapsed profile of the stent graft and increase the stiffness during delivery to the artery.
Another approach to the treatment of aneurysms is the use of a porous tubular stent graft comprised of one or more layers of braided metal. In this approach, the tubular braid is placed directly against the aneurysm before the aneurysm has become dangerous in size. The braid has a maximum expansion diameter matched to the aneurysm maximum diameter and the stent graft incorporates into the wall of the aneurysm by tissue ingrowth, thereby strengthening the wall and inhibiting any further growth of the aneurysm.
Another application of stents or stent grafts is for treating a dissection of a vessel such as, for example, the thoracic aorta, whereby a tear in the vessel lining threatens to cause an aneurysm if not treated. In such cases, the tear may allow blood flow against the adventitial layer of the vessel and must be sealed. A good seal must be achieved between the vessel wall and the stent or stent graft to ensure that blood cannot enter the origin of the dissection. On either side of the origin of the dissection, the stent graft may be more porous as vascular support is the primary attribute for the remainder of the stent graft. A stent graft may achieve the seal by addition of a polymer or textile fabric but this adds to the device delivery profile.
Metallic, “super-elastic,” braided, tubular members are known to make excellent vascular occlusion, restrictor, and shunt devices, for implant within the body. These devices are typically braided from filaments of Nitinol and subsequently heat set to “memorize” a final device shape. Such devices may be elongated for delivery through a catheter to a treatment site, and upon removal from the delivery catheter, may self-expand to approximate the “memorized” device heat set shape. The devices have various shapes designed to occlude, restrict flow, or shunt flow to various parts of the vascular anatomy by restricting or diverting blood flow through all or a portion of the device. Since the devices are subjected to blood pressure, there must be sufficient retention force between the device and the vascular wall to prevent device dislodgement.
Therefore, it would be advantageous to provide a medical device having increased radial strength while retaining a small profile and flexibility for delivery to a target site. It would also be advantageous to provide a medical device capable of being sufficiently anchored at a target site and effectively treating the target site.